This invention relates to the control of power and traffic levels of transmitted signals in telecommunication systems, in particular spread spectrum, or code division multiple access (CDMA) systems.
In a typical CDMA system, an information data stream to be transmitted is impressed upon a much-higher-bit-rate data stream produced by a pseudo-random code generator, such that each information signal is allocated a unique code. A plurality of coded information signals are transmitted as modulations of radio frequency carrier waves and are jointly received as a composite signal at a receiver. Each of the coded signals overlaps all of the other coded signals, as well as noise-related signals, in both frequency and time. By correlating the composite signal with one of the unique codes, the corresponding information signal can be isolated and decoded.
In a mobile radiotelephone system, interference between different call connections using the same radio channel can be reduced by regulating the transmission power levels of mobile stations and base stations in the system. Preferably, only the transmission power necessary to maintain satisfactory call quality is used to reduce the likelihood of interference between calls using the same radio channel. An attribute such as a signal-to-noise interference ratio (SIR) can be used as a measure of quality or power of received signals in the system, i.e., as one measure of call quality in the system. The each connection between a mobile station and a base station can have a SIR, and SIR measures of different connections can be different.
By regulating power in communication systems that use CDMA so that only the transmission power necessary to maintain satisfactory call quality is used, capacity of the system can be increased by approximately 70% as compared with an unregulated system where signal transmission power is unnecessarily large, assuming that all of the calls or connections have the same SIR target or required SIR. In addition, mobile stations in the system consume less energy when transmit power levels are maintained at a lowest possible level. Accordingly, batteries used to power mobile stations can have a smaller capacity, allowing the mobile stations to be lighter in weight and smaller in size.
One known type of power control is so-called "fast" SIR-based control. The basic principle of fast SIR-based control of signal transmission power is that under normal conditions, an increase in signal transmission power will cause a corresponding increase in SIR. In fast SIR-based control, when the SIR of the signal transmission is higher than necessary, the signal transmission power is decreased. When a SIR of a signal transmission from a mobile station to a base station is too low, the signal transmission power of the mobile station is increased. Precise details regarding fast-SIR based control of signal transmission power in CDMA systems will be apparent to those skilled in the art, and are not discussed in this document.
When a mobile communication system is overloaded, signal transmissions within the system can mutually interfere. In such a scenario, increasing signal transmission power does not effectively increase the SIR because of "party effects".
The party effect phenomenon is similar to what happens at a party when people talking with each other speak loudly to hear over others who are speaking loudly, thus causing the overall noise level to become large. Specifically, in a system employing fast SIR-based control of signal transmission power, a first mobile station experiencing a SIR that is below a SIR target value or threshold will increase signal transmission power to bring the SIR to the target value. If the interference that the first mobile station is trying to overcome is caused by signal transmissions from a second mobile station, and signal transmissions from the first and second mobile stations are mutually interfering, then the signal transmission power increase by the first mobile station can cause a corresponding increase in interference with the second mobile station's signal transmission and degrade the second mobile station's SIR below its target value. In response the second mobile station will increase its signal transmission power to increase its SIR, thus exacerbating the original problem. Positive feedback is present in the system, and the mobile stations will each increase signal transmission power until maximum power levels are reached, without achieving the desired quality or power of the received signals. Party effects arising in one cell of the system can spread to neighboring cells in the system when, for example, high signal transmission power levels in the one cell interfere excessively with signal transmissions in an adjacent cell.
U.S. Pat. No. 5,574,982 to Almgren et al ("Almgren") provides a solution to avoid party effects in cellular radio communication systems. Almgren is incorporated by reference into this document. Almgren describes monotonically reducing a target carrier-to-interference ratio (C/I) or SIR as signal transmission power is increased. Thus, as signal power is increased to compensate for interference, the allowable level of interference is also increased. In effect, the system avoids the large reduction in signal quality of all users in the same cell that would result from party effects by tolerating a smaller reduction in signal quality, i.e., by increasing the allowable level of signal interference. However, although party effects are avoided, signal quality is nevertheless reduced.
Problems can arise, however, in a system having a slow, quality-based power control in addition to a fast, closed loop SIR-based power control that uses the method described in Almgren. In such a system, when an experienced signal quality (typically, a frame error rate) decreases below an acceptable value, e.g., when the system is overloaded, the slow power control will increase the SIR target by increasing the value of a specified SIR threshold. In contrast, the fast, SIR-based power control will effectively decrease the SIR target as signal power is increased to remedy the reduction in signal quality. Thus, the system suffers from the disadvantage that the slow power control and the fast power control can counteract each other because they both alter the SIR target in different directions. Consequently, the fast power control can only temporarily stabilize the system.
M. Andersin, in "Power Control and Admission Control in Cellular Radio Systems", Ph.D. thesis, Royal Institute of Technology, Stockholm, Sweden, May 1996, describes a system wherein control of signal transmission power is integrated with removal of signal connections. The integration is achieved by performing fast, closed loop SIR-based control of signal transmission power only for supported signal connections, and deactivating all other (i.e., unsupported) signal connections by setting their respective signal transmission powers to zero. A supported signal connection is a signal connection whose SIR target can be achieved within the system. In contrast, an unsupported signal connection is a signal connection whose SIR target cannot be achieved within the system. Connection removal algorithms are used to select specific connections which can be removed to stabilize the system.
In other words, the SIR-based power control algorithm is not altered based on signal traffic congestion levels within the system. Instead, the set of signal connections controlled by the SIR-based power control algorithm is altered. The connection removal algorithm removes signal connections until remaining signal connections can each achieve their SIR targets under the control of the SIR-based power control algorithm. Such an integrated control will cause a communications system to a) require additional signaling over the air interface, since knowledge of the output power of the mobile stations is needed; b) require more complex infrastructure for the mobile stations and the base stations since the fast closed loop power control cannot be applied when deactivating some connections; and c) be unable to protect real time services in the deactivation phase.